With regard to a centrifugal blower, a centrifugal fan is generally housed in a scroll casing. The scroll casing includes a suction scroll wall, a motor scroll wall that is parallel with the suction scroll wall, and a peripheral wall connecting the peripheries of the suction scroll wall and the motor scroll wall. The suction side wall defines a suction port in a central position and forms a bell mouth on the periphery of the suction port. A motor is fixed to the central portion of the motor scroll wall. Also, the scroll casing forms a scroll passage between the periphery of the centrifugal fan and the peripheral wall. A radial dimension of the scroll passage increases toward an air downstream position. The scroll casing forms a blowing outlet at the downstream position of the scroll passage. The blowing port connects to a blowing duct. Further, the blowing duct connects to an air conditioner duct of an air conditioner unit.
With regard to a centrifugal blower disclosed in JP-A-5-195995, as shown in FIG. 12, a scroll passage 5 consists of a blowing passage 50 and a protruded passage portion 5B to restrict pressure loss and to reduce noise problem. Specifically, a scroll wall is protruded in an axial direction at a position corresponding to the scroll passage 50 so that the protruded passage portion 5B is formed.
Further, as shown in FIG. 13, the protruded passage portion 5B is proposed to have a substantially rectangular-shaped cross-section to facilitate creation of a secondary flow D2 in the scroll passage 5. In this case, the pressure loss and the noise are reduced within a wide air blowing region where the flow rate of the centrifugal blower 1 changes. However, within the scroll passage 5, the discharging flow D1 from the centrifugal fan 4 and the secondary flow D2 are likely to interfere with each other at positions proximate to the discharging ports 45 of the centrifugal fan 4, which are formed on the periphery of the centrifugal fan 4. The interference of the primary flow D1 and the secondary flow D2 may result in an increase in noise.